Writing on the Sky Vault and on Clay Tablets

At every point of history, mankind has made the best of what the environment had to offer for living. When the conditions changed, like during the ice ages, human cultures adopted new ways of living as a response to those changes. Sometimes unexpected things resulted. An example is the formation of the fertile delta region between the Euphrates and Tigris rivers flowing into the Persian Gulf. When the surface of the Gulf gradually rose tens of meters after the Ice Age, the flow of the two rivers slowed making the region good for farming. However, when the climate got dryer around 3500 BC, large scale irrigation became important and power became centralized in Sumerian cities. Life was centered on the temple, dedicated to the god of that city. The temples were large administrative and economic centers, headed by the clergy. The polytheistic religion of Sumer was inherited by Babylonia around 1500 BC.

Writing had been invented around 3000 BC by Sumerians. It started a flow of unexpected cultural evolution. The art of cuneiform writing was originally useful for bookkeeping in the economic centers, temples, but it gradually found application in many other fields than business, including sky watching. How celestial bodies move gives us both ancient and modern methods of timekeeping. We know that Sumerian clergy tracked the Moon to build a lunar calendar by recording the information on clay tablets.

However, their direct descendents, the Babylonian priests, were instead curious to learn what signs the divine celestial stage offered about the future of the rulers and the kingdom. The sky formed a huge screen with "texts" that the specialist tried to interpret. Thus, systematic astrology was born, together with a developed state. Interest in the misty future was strong and there were also other methods of prediction, like watching the flight of birds. In contrast to today, at that time astrology was quite a rational undertaking when stars were viewed as gods or their representatives. It was logical to try to find links between celestial phenomena and earthly happenings. Some were indeed known: the seasons are marked by the path of the Sun among the stars and tides obey the Moon. With little artificial light to block their view, the ancients were much more observant of the sky than most people today.

In Mesopotamia, a lunar calendar was based on the phases of the Moon. Each month began on that evening when the thin sickle of the growing Moon was first seen after sunset. Nowadays, the solar calendar (which is consistent with the seasons) dominates everyday life, but the lunar calendar is still important for religious purposes.

Because of the yearly cycle of the Sun, different constellations are visible in the evening at different seasons. The appearance of the sky today is almost the same as thousands of years ago. Many constellations still carry the names that shepherds or seamen once gave them. Certainly the starry patterns initially had real meaning. Various animals, gods, and mythical heroes were permanently etched on the sky. But the constellations also form a map that helps one to identify the place where something happens in the sky. In modern astronomy, there are 88 constellations with definite borders. For instance, when comet Halley last appeared, one could read in the newspaper that in December 1985 the visitor would be in the constellation of Pisces just south of Pegasus. With this information it was easy to spot the famous comet through binoculars. The daily motion of the Earth merely caused the comet and the constellation to move together across the sky, keeping their relative positions.

The Babylonian astrologers were well aware that not all celestial objects move faithfully together with the stars. The Moon shifts about 13° (or 26 times its own diameter) eastward relative to the stars every day. It takes a little more than 27 days for the Moon to come back roughly to the same place again among the stars. Also the Sun moves relative to the stars although the glare blots them out. However, during the year, different constellations are visible near the Sun just before sunrise or a little after sunset. Thus it was deduced that the Sun moves around the sky visiting the same constellations through the year. Astrologers divided its route, or the ecliptic, into 12 equal parts and the Sun stayed in each for about one month. These constellations came to define the signs of the zodiac. The word ecliptic means the solar path where the eclipses occur.

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The Art Of Astrology

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